My parents think my bisexuality is a phase how do I tell them it's not?

The best thing to do is to tell them it is not a phase and that they should support you. If they insist it is, which they will, tell them that even if it is a phase, I want you to support me and my decisions, now, here. Tell them not to wait until they are persuaded that your Bi to support you, they should support you now.